In many technical fields there is a need for measuring and imaging the tomography and/or the topography of transparent or semi-transparent objects. There are many optical methods for accomplishing at least part of this task, such as the structured-light scanning method, confocal microscopy, phase shift method, optical coherence tomography or holography. In some methods, the chromatic aberration of the optical system can prevent accurate measurement while some methods may advantageously exploit the chromatic aberration. For example, in the structured-light scanning methods, dispersion or chromatic aberration can lead to blurring of the structured-light pattern, which reduces the resolution. Alternatively, in chromatic confocal microscopy, a suitable objective lens having multiple focal lengths that are dependent on wavelength is used to image a point or an array of point sources of wideband-spectrum light. As a result, depending on the wavelengths of the components of the illumination source emanating from the point sources, the illuminated point sources are imaged to different image planes having a range determined by the minimum and maximum wavelengths of the wideband illumination source.
An object located within this range reflects the light at each image plane, the reflected light being re-imaged into the pinhole or pinhole array by the objective lens. The intensity of the reflected light depends on the extent to which the image plane and the points of the object's surface that reflect the light coincide. Specifically, if an image plane coincides with points of the object, which may lie on one or more surfaces of the object, then the intensity of the light reflected by these points of the object will be maximal. The maximum intensity is detected for the particular wavelength that is imaged in focus at a particular point on the object's surface or one of its layer's surfaces. Thus, by determining the spectral peak position, the distance of the object's surface or of its layer's surfaces to the objective lens at this point and hence the object's tomography and/or topography may be determined. The evaluation is performed point-by-point using a spectrometer or line-by-line using a line spectrometer with camera chip. In particular, the multi-focus arrangement, preferably in combination with a micro-lens array and a matched pinhole array, is a promising option on account of the low expected time requirements for image recording.
U.S. Pat. No. 8,515,528 describes such a measuring arrangement and method for the three-dimensional measurement of at least part of an object includes a light source with a continuous spectrum, a device for generating a multifocal lighting pattern, a lens with a large chromatic aberration for imaging foci of the lighting pattern on the object, a detection unit for generating the wavelength spectrum of the foci that are confocally imaged on the object via the lens, and a spectrum-dispersive device disposed between the confocally imaged foci and the detection device.
EP-B-0 321 529 discloses a measuring arrangement for measuring the distances between an objective lens with high chromatic aberration and an object. A black-and-white CCD camera is used as detector, in front of which is arranged a spectrum-dispersing apparatus with an input slit. The wavelength information for each point is converted to location information to obtain a profile image of the surface of the object.
EP-B-0 466 979 discloses an arrangement for confocal and simultaneous picture generation with a moving pinhole diaphragm in the illumination ray path with a position-measuring system, a raster camera and a processing device for the raster camera, which synchronously reads out the pixels of the pinhole diaphragm.
US 2004/109170 discloses a sensor for rapid optical distance measurement based on the confocal imaging principle. The sensor includes a light source, which emits an illuminating light with different spectral components, and an optical imaging system, through which the illuminating light is directed onto the surface of a measurement object. Different spectral components of the illuminating light are focused at different distances from the optical imaging system due to a chromatic aberration of the optical imaging system. Also provided is a beam splitter, arranged so that measuring light reflected back at least partially from the surface, is separated spatially from the beam path of the illuminating light. Further, a light receiver is included, which records the measuring light, separated spatially from the beam path of the illuminating light, with spectral resolution. Finally, an analysis unit determines the distance between the sensor and the surface from the intensities of measuring light recorded for different spectral components. Such a system requires a pinhole for point by point imaging and a scanner that scans each image point to construct a 2D image.
DE-A-103 21 885 is also a confocal measuring arrangement for the three-dimensional measuring of an object with chromatic depth resolution, in which a multitude of foci are generated by means of a micro-lens array and are imaged onto the object. The reflected light is focused back into the plane of the micro lens foci. This arrangement is used to measure two or three-dimensional micro-profiles of a test objects of two or three-dimensional profiles of reflectivity or transparency.
It is thus known in the art to determine the distance between the sensor and the surface from the intensities of measuring light recorded for different spectral components.